Google has good guidance on this:
Where to place your Wifi devices - Google Nest Help
Troubleshoot slow internet on Google Nest Wifi or Google Wifi - Google Nest Help
One thing that's not explicitly stated is you should aim to place your primary Wifi point as close as possible to the centre of your home. That's because the Google Wifi and Nest Wifi mesh system uses a fewest hops approach. If one of your secondary points can get even a weak connection to your primary Wifi point, it will do that instead of going through an intermediary Wifi point that may physically be closer, which results in slower speeds for your devices. That's why not installing them in a straight line, and placing your primary Wifi point close to the centre of your home really helps, along with all the other guidance in that article above. If you still experience issues after that, you may want to look at hardwiring all your Wifi points, but if you're going to do that hardwire all of them, not just some of them, as partial or disjointed setups where only some are hardwired create problems that are very hard to diagnose.
Hardwire Wifi devices with Ethernet - Google Nest Help
One other thing I will say is Google don't recommend having more than 5 Wifi points in a single network. The reason for this is the same reason I mentioned above. If you really need 7 points for coverage purposes, a hardwired connection to each of your points would be significantly more reliable. If that's not possible, it's essentially a must to have your primary Wifi point centrally placed. Typically, it just won't be enough if the most distant Wifi points are far away and if you really need that many Wifi points, only wiring them is going to make the performance in that kind of setup satisfactory.
